A number of different types of light sources ate used to provide general lighting. General lighting for offices, residences or similar has been provided by incandescent lighting or fluorescent light in the past. One of the reasons for favouring such light sources is the improved colour rendering that can be provided by light sources that produce light over a broad bandwidth.
Solid state lighting usually involves the use of LEDs hat, upon application of a relatively constant forward current, produce a relatively stable quantity of light at a particular dominant wavelength. Although such light sources have advantages in reliability and efficiency over other light sources such as incandescent lighting, they have generally been unable to provide the variety of wavelengths necessary to produce white light cost effectively.
To produce white light from LEDs, it is possible to combine two or more LEDs having different dominant wavelengths that complement each other to produce white light. However, the cost of each individual light source is increased with each additional different coloured LED provided. Although white light can be produced by as few as two complementary LEDs, such light will not produce good colour rendering as many of the wavelengths from the spectrum of light are missing or insufficiently provided to provide accurate colour rendering over a reasonable range.
Various attempts have been to produce white light from a single LED or to alter the wavelength of light generated from LED sources.
One category of solutions has sought to alter the wavelength of the light emitted by the lamp subsequent to its production by the LED through the use of surrounding apparatus. This may include the use of an LED to stimulate a phosphor-based colour conversion system as is described in U.S. Pat. No. 6,028,694 or the use of a surrounding reflector to alter the emitted wavelength as is described in U.S. Pat. No. 5,743,629. The difficulty with such methods is that it requires fair more expensive production or installation for the individual light sources as each LED or a small group of LEDs requires provision with additional items to produce the desired light, With the phosphor based solutions, the use of phosphor Creates extra variation in the light produced by each LED, A consistent colour between LEDs is difficult
Another method to try and produce some variation in emitted wavelengths involved producing LEDs having a graded band gap active region that is graded between two materials of different centre wavelengths of light emission. Such an LED is disclosed in U.S. Pat. No. 5,045,896. However, the production of such LEDs is difficult to provide any accuracy in the emissions of the light source.